Marcus ends the Meditations with an envoy—a kind of postscript that bids farewell and sends the reader/writer out into the world. Since Marcus never intended his reflections to see the light of day, he's really just addressing himself when he opens the last chapter in this way:
Mortal man, you have lived as a citizen in this great city. (12.36)
Marcus uses the image of the city throughout his work to identify himself with something even greater than Rome. By using the image here, he's talking about his existence at both the macrocosmic level (he's citizen of the world and the universe) and the microcosmic (he's a resident of his own body). But now the universe is giving him his marching orders, and he will have no choice—despite his 50 years of residency—to give up his place in the city.
Marcus might be close to his death here, but he's certainly not on his deathbed. He's simply practicing some of his best philosophical principles when he focuses on his future death. In the process, we see in brief some of the ideals he laid out in the body of the Meditations.
Marcus uses the simile of life as a comedic play to remind himself that life may end at any time (a play can be over in three acts instead of five)—and that there's no fighting with the order to leave.
Marcus closes his reflections by focusing on a crucial tenet of Stoic philosophy: personal peace. He tells himself to be at peace with his coming death because it is man's nature to fade. Therefore, all is right with the world.